1. Field of the Invention
This invention relates to an injection mold for use in the manufacture of golf balls. More particularly, it relates to an injection mold for golf balls defining a spherical cavity therein wherein a plurality of gates open to the cavity are three-dimensionally arranged so that golf balls having a cover of a uniform gage can be molded without short shots.
2. Prior Art
In the prior art, two-piece solid golf balls are customarily prepared using injection molds. Referring to FIGS. 11 and 12, a method for injection molding a cover around a core is described. FIG. 11 is a cross sectional view of an injection mold defining a spherical cavity, and FIG. 12 is an imaginary perspective view of the spherical cavity of the mold. The injection mold 1 for molding a cover around a core includes an upper mold section 1a and a lower mold section 1b having inner walls which define a spherical cavity 2 when the mold sections are mated at a parting plane P. The inner walls are provided with dimple-forming protrusions (not shown). It is noted that the parting plane or line P between the upper and lower mold sections 1a and 1b corresponds to an equator plane or line of the cavity. A plurality of support pins 4 are vertically extended through the upper and lower mold sections 1a and 1b (four pins in each of the upper and lower mold sections in FIG. 11). A plurality of gates 5 (eight in FIG. 11) are disposed along the parting line P (or near the parting line P in the case of tunnel gates). In preparing a golf ball, a core 3 is placed in the cavity 2 as an insert. The core 3 is held in place by the support pins 4. A cover molding material is injected into the space between the core 3 and the cavity-defining wall through the gates 5. Immediately before the cover molding material is injected or at the same time as the completion of injection, the support pins 4 are withdrawn until their distal ends are flush with the cavity-defining wall. In this way, the core 3 is enclosed with a cover having a multiplicity of dimples. It is noted that in FIG. 11, a pin 6 is received in a hole 7 to define a venting space 8 at each of opposite poles.
When the cover molding material is injected into the space between the core 3 and the cavity-defining wall through each gate 5, the material diffuses and moves radially along the cavity-defining wall as shown by arrows in FIG. 12 and eventually converges at the deepest position of the cavity (in proximity to the north and south poles). Of the flow distances that the molding material moves, the length designated L between the gate and the pole (north or south pole) is the longest. In this example, this maximum flow distance L is approximately equal to a quarter of the circumference of the spherical cavity, that is, 2.pi.r/4.
Longitudinal boundary lines (or imaginary weld lines) U indicate the positions at which the molding material injected from adjacent gates merge with each other after the completion of injection. The surface area that is covered by the molding material admitted through each gate is the surface area circumscribed by two adjacent longitudinal boundary lines U, that is, the hatched region in FIG. 12. In this example, this surface area coverage is approximately equal to or greater than 1/8 of the overall surface area of the cavity.
In the prior art injection mold 1 mentioned above, however, a plurality of gates 5 (eight in the illustrated example) are equidistantly spaced along the parting line (or equator line) P between the mold sections. Even when tunnel gates are employed, they are disposed in close proximity to the parting line P for the convenience of gate cutting. As a result, an intense injection pressure of typically 400 to 2,000 kg/cm.sup.2 or more is applied to the core in a perpendicular direction, thereby deforming the core to expand it in polar directions into a rugby ball shape. Consequently, the spaces at the deepest positions of the cavity become narrow so that the molding material may not fully penetrate thereto, resulting in short shots. Particularly when it is desired to mold a thin cover (with a gage of less than 1 mm), the injection pressure must be higher and as a consequence, the space between the core and the cavity wall is substantially blocked. Molding is no longer possible.
Moreover, since the gates are disposed along the parting line P, the maximum flow distance L of the molding material becomes longer. In addition, the surface area coverage of the molding material from each gate also becomes larger, requiring a longer time for filling. Then the molding material can lower its fluidity before the completion of filling, failing to mold a cover of a uniform gage. Particularly near the polar positions where the molding material converges, the molding material can substantially lower its fluidity and even solidify before filling, resulting in molding failure.